A dynamic microphone has favorably been used mainly as a vocal microphone. Unfortunately, the dynamic microphone is liable to pick up handling noise (touch noise) because the mass of an oscillation system including a diaphragm having a voice coil is large. Therefore, many of the dynamic microphones have a vibration-proof structure for reducing handling noise.
As described in Non-patent Document 1 (“Analysis of Touch Noise of Vocal Dynamic Microphone” by Tukasa Takeshita et al., Transactions of The Acoustical Society of Japan (October, 1993), pp. 539-540), as typical examples, the vibration-proof structures come in two types: a floating type in which a unit with a back air chamber is supported by an elastic body such as rubber to make vibrations of a grip (microphone casing) less liable to be transmitted, and a canceling type in which an inertia force generated on the diaphragm by vibrations is canceled by a pressure developed in the back air chamber.
Of these types, the latter canceling type is theoretically a type effective in reducing touch noise; however, it is said that this type is difficult to achieve the vibration proofing effect as expected theoretically in mass production because the parameter setting requires high accuracy.
The vibration-proof structure of the dynamic microphone of the present invention is of the former floating type. Therefore, the configuration and problems of a dynamic microphone of a conventional example provided with a floating type vibration-proof structure are explained with reference to schematic views of FIGS. 4 to 6.
Referring to FIG. 4, a dynamic microphone 1B of the conventional example includes, as a basic configuration, a microphone body 10 and a microphone casing 20 used as a microphone grip.
The microphone body 10 includes a microphone unit 110 and an inner cylinder 120 having a back air chamber 121 for the microphone unit 110 therein. As shown in FIG. 5, the microphone unit 110 consists of a diaphragm 111 and a magnetic circuit part 115.
The diaphragm 111 has a center dome 112 and a sub dome (also referred to as an edge part) 113 integrally provided around the center dome 112, and the whole of the diaphragm 111 is formed of a synthetic resin film. On the back surface side of the diaphragm 111, a voice coil 114 is integrally attached to the boundary portion between the center dome 112 and the sub dome 113 with an adhesive or the like.
The magnetic circuit part 115 includes a disc-shaped permanent magnet 116 magnetized in the thickness direction, a pole piece 117 formed into a disc shape like the permanent magnet 116 and arranged on the one pole side of the permanent magnet 116, a bottomed cylinder-shaped yoke body 118 arranged on the other pole side of the permanent magnet 116, and a ring yoke 119 arranged at the opening end of the yoke body 118 with a magnetic gap G being provided between the ring yoke 119 and the pole piece 117.
In this example, on the outer periphery side of the ring yoke 119, a flange 119a is formed to support the peripheral edge portion of the sub dome 113. For the diaphragm 111, the peripheral edge portion of the sub dome 113 is supported by the flange 119a of the ring yoke 119 so that the voice coil 114 can oscillate in the magnetic gap G.
The inner cylinder 120 consists of a bottomed cylindrical body made of a metal or a synthetic resin, and the opening side thereof is airtightly connected to the magnetic circuit part 115 side of the microphone unit 110 coaxially with the microphone unit 110. Although not shown in the figures, a vent hole is formed in the bottom portion of the yoke body 118, and the back air chamber 121 of the inner cylinder 120 communicates acoustically with an air chamber on the back surface side of the diaphragm 111 via the vent hole.
The microphone casing 20 has an inside diameter larger than the outside diameter of the inner cylinder 120, and consists of a cylindrical body serving as an outer cylinder for housing the inner cylinder 120 therein. Usually, the microphone casing 20 is manufactured from a metallic material such as a brass alloy. Although not shown in the figures, an output connector is mounted in the bottom portion of the microphone casing 20.
For the microphone body 10, the inner cylinder 120 side thereof is housed in the microphone casing 20 in such a manner that the microphone unit 110 is arranged on the outside of the microphone casing 20. According to the floating type, to reduce handling noise, the inner cylinder 120 is supported coaxially in the microphone casing 20 via an elastic member (shock mount member) 30.
In this example, as the elastic member 30, two elastic members of a first elastic member 31 and a second elastic member 32 are used. In most cases, as both the elastic members 31 and 32, rubber elastic bodies each formed into a ring shape are used. The elastic member 30 is interposed between the outer peripheral surface of the inner cylinder 120 and the inner peripheral surface of the microphone casing 20 in a state of being compressed moderately.
The first elastic member 31 and the second elastic member 32 are arranged at a predetermined interval along the axis line direction of the inner cylinder 120. In this example, the first elastic member 31 is arranged at a position close to the lower end side of the inner cylinder 120, whereas the second elastic member 32 is arranged at a position close to the upper end side of the inner cylinder 120, so that the inner cylinder 120 is supported at two locations.
In FIG. 5, the oscillation direction of the diaphragm 111 (the axis line direction of the voice coil 114) is taken as the principal axis direction Y of the microphone indicated by an arrow mark Y, and the direction intersecting at right angles with this principal axis direction Y, which is indicated by an arrow mark X, is shown as the direction X perpendicular to the principal axis of the microphone. Because the diaphragm 111 has a structure such that the peripheral edge portion of the sub dome 113 is supported by the flange 119a of the ring yoke 119, the diaphragm 111 scarcely moves in the direction X perpendicular to the principal axis of the microphone.
However, as reported in Non-patent Document 1 as well, actually, vibration noise occurs even in the case where vibrations are applied to the dynamic microphone 1B from the direction X perpendicular to the principal axis.
The cause for this is as described below. The microphone unit 110 includes the members each having a large mass, such as the permanent magnet 116, the pole piece 117, the yoke body 118, and the ring yoke 119. For this reason, the center of gravity O of the microphone body 10 exists on the upper side (the microphone unit 110 side) of the supporting point S of the elastic member 30 (the first elastic member 31). Therefore, as shown in FIG. 6, when vibrations are applied from the direction X perpendicular to the principal axis, the microphone body 10 rolls in the direction indicated by an arrow mark θ.
Accordingly, an object of the present invention is to provide a dynamic microphone in which vibration noise generated by the rolling of a microphone unit caused by a vibration component perpendicular to the principal axis direction of the microphone is reduced effectively.